1. Field of the Invention
The present invention relates to an organic light-emitting display screen and more particularly to an organic light-emitting display screen of the active matrix type, or AMOLED (Active Matrix Organic Light Emitting Diode).
In an organic light-emitting display screen, the picture element (pixel) is an organic light-emitting diode structure. Such a display screen does not require any additional light source, in contrast to other display devices such as the devices referred to as LCDs (Liquid Crystal Displays). Its other advantages include a low power consumption, a high brightness and low fabrication costs. The display of video data by OLED diodes is based on the principle of the modulation of the diode current. This is achieved by a current-driver transistor, which receives a voltage on its gate corresponding to the video data to be displayed and supplies a corresponding current to the diode.
2. Discussion of the Background
In FIG. 1 is shown an equivalent circuit diagram of an OLED picture dot or pixel according to the prior art. As illustrated in this Figure, the device usually comprises a crossed array of select lines S1, S2, . . . Sm, and data lines D1, D2, . . . Dn, m and n being integer numbers. To each pixel Pixi,j of the matrix corresponds one select line Si, iε[1, . . . m], and one data line Dj, jε[1, . . . n], by which this pixel is controlled. Each pixel Pixi,j comprises an organic light-emitting diode or OLED and an associated current-driver circuit. This circuit comprises a switching transistor T1, a holding capacitor C1 and a current control, or “driving”, transistor T2. A gate electrode of the switching transistor T1 is connected to the corresponding select line Si and a conduction electrode, source or drain, connected to the associated data line Dj. The other conduction electrode is connected to one terminal of the holding capacitor C1, and to the gate electrode of the driving transistor T2. The holding capacitor C1 has its other terminal connected to a reference voltage VDD. The driving transistor is connected in series with the organic light-emitting diode OLED between the reference voltage and ground: the anode of the diode is thus connected to a conduction electrode (drain or source) of the transistor T2 and its cathode is connected to a potential VK common to all the diodes of the display screen, typically the electrical ground. In the example, the transistors T1 and T2 are n-type.
In the usual manner, at each new video frame, the rows of pixels are selected in sequence by the application to their respective select line S1, S2, . . . Sm, of a selection voltage Vgon, lasting for a row time. The video data signals corresponding to a selected row of pixels are applied to the data lines D1, . . . Dj. These selection and data lines are controlled by respective driver circuits, called row driver and column driver, which may be integrated into the matrix or external to it. These circuits are well known to those skilled in the art.
The picture element Pixi,j is now considered. When the select line Si is addressed, the switching transistor T1 turns on for the addressing time (row time) of the line. It switches the video voltage present on the data line Dj onto the gate of the driving transistor T2. The transistor T1 then turns off and isolates the pixel from the data line. The capacitor C1 then ensures that the voltage on the gate of the transistor T2 is maintained. The transistor T2 operates as a controlled current source: it supplies to the OLED diode a current whose intensity depends on the video voltage switched onto its gate. The OLED diode emits a corresponding intensity of light. In this operation, the transistor T2 is continuously supplied with power: the duty cycle for the application of this voltage is therefore 100% for each video frame. The diode is also continuously driven, with a duty cycle of 100%.
The intensity of the current flowing in the driving transistor T2 depends on the level of the voltage switched onto the gate of the transistor T2. It also depends on the threshold voltage of this transistor. It is recalled that the threshold voltage of a transistor represents the minimum potential difference that must be applied between the gate and source of the transistor so that the latter allows current to flow: below this, the transistor is said to be turned off. The higher the potential difference, the more current the transistor allows to flow, until it becomes saturated. The drain-source current Ids is given by the following general equation: Ids=K(Vgs−Vth)2, where Vth is the threshold voltage and Vgs the gate-source voltage.
In order to have a sufficient luminance and a good uniformity of the display screen, the current Ids corresponding to a given grey level must be constant over time whichever pixel of the display screen is considered.
The invention relates more particularly to AMOLED display screens, whose transistors of the active matrix (the transistors T1 and T2 of the pixels Pixi,j) are thin-film transistors, referred to as TFTs, and notably to AMOLED display screens using an active matrix with amorphous silicon TFT transistors, which matrices are advantageously inexpensive. In these display screens, a significant positive drift of the threshold voltage of the driving transistor T2 is observed with the level of the voltage applied continuously to its gate (duty cycle of 100%). More generally, the threshold voltage of these transistors varies with temperature, the gate-source voltage applied to it and the duty cycle, in other words the time during which the voltage Vgs is applied with respect to the frame time. This also applies to other types of transistors, for example transistors using materials between amorphous silicon and polycrystalline silicon.
FIG. 2 illustrates a typical curve 1 of the drain-source current Ids as a function of the gate-source voltage Vgs of an amorphous silicon TFT transistor, under initial conditions, with a threshold voltage Vth0. In the example, the transistor is an n-type transistor. Such a transistor has a positive or zero threshold voltage. It is made to conduct by applying a positive gate-source voltage, higher than its threshold voltage. After the transistor has been subjected to a positive stress SP, in other words to a gate-source voltage higher than the threshold voltage of the transistor, during one frame time (typically 20 milliseconds), a second curve 2 is obtained, which corresponds to a translation of the initial curve in the direction of increasing Vgs. This means that, owing to the positive stress, the threshold voltage of the transistor has increased, going from Vth0 to Vth1>Vth0. The effect of this is that, for the same gate-source voltage, a lower current Ids will be driven in the transistor, and therefore the OLED diode will emit less light.
For a given data value to be displayed, the level of light obtained is therefore variable according to the effective threshold voltage of the transistor, at the time of observation. Since the threshold voltage has a positive drift, the current delivered by the driving transistors decreases, which results in a loss of luminance on AMOLED display screens.
Since the video voltages to be displayed vary from one pixel to another, this variation of the threshold voltage of the driving transistors T2 furthermore results in a significant non-uniformity over the AMOLED display screen.